This invention relates to a hydraulic control device for an automatic vehicle transmission and is of the type which has a main pressure control valve for controlling an engine load-dependent main output pressure for actuating the switching components of the transmission. The hydraulic control device further has a pressure-limiting valve which delivers an upwardly limited control pressure dependent upon the engine load. The control piston of the pressure-limiting valve is exposed to the control pressure acting against the force of a spring.
A hydraulic control device of the above-outlined type is known and is disclosed, for example, in German Auslegeschrift (Accepted Published Patent Application) No. 2,449,382. In the hydraulic control device described therein, the pressure-limiting valve applies to the main pressure control valve a control pressure which, up to a predetermined load condition, is in essence proportionate to the engine load and which, dependent upon the vehicle speed, is upwardly limited by two different maximum pressure values. For this purpose, there is provided a switch-over valve which, dependent upon the vehicle speed, causes an additional work face of the control piston of the pressure-limiting valve to be exposed to pressure or, as the case may be, to be depressurized.
Although the above-outlined known control device sets a higher main pressure for the higher load ranges at vehicle speeds which are lower than, for example, 30 km/h, no changes in the main output pressure are obtained in response to partial loads. Since, however, upon starting a vehicle equipped with an automatic transmission, the torque is increased up to 2.5-fold of the engine torque because of the hydrodynamic torque converter, precisely in the partial load range, the main output pressure which actuates the switching components of the transmission should be increased for the purpose of preventing a slip-through of a switching component.